Feed forward amplifier

ABSTRACT

In a feed forward amplifier, an RF amplifier is supplied with an input RF signal at an input terminal for amplifying the same; a distortion extraction loop supplied with the input RF signal and further with the output RF signal from the RF amplifier is for extracting non-linear distortion components formed in the output RF signal as a result of amplification in the RF amplifier; a variable phase shifter is provided in the distortion extraction loop for varying a phase of the input RF signal; a variable attenuator is provided in the distortion extraction loop for attenuating an amplitude of the input RF signal that has been supplied to the distortion extraction loop; and a distortion extraction circuit is provided in the distortion extraction loop for producing a distortion output signal that includes non-linear components; further, a control circuit is supplied with the input signal and with the distortion output signal for extracting a main signal component contained in the distortion output signal. The control circuit controls the variable phase shifter and the variable attenuator such that a ratio of the main signal component level with respect to the input RF signal level is decreased.

TECHNICAL FIELD

The present invention generally relates to radio telecommunicationsystems and more particularly to a feed forward amplifier suitable for abroadband RF amplifier used in base stations of mobile telephone systemsas well as to a control of such a feed forward amplifier.

BACKGROUND ART

With the deployment of digital mobile telephone systems, there is ademand for compact, low-power consumption base stations that can beconstructed at a reduced cost. In order to achieve this object, a commonamplification construction is employed for the RF amplifiers of the basestations, wherein a plurality of carriers of different frequencies areamplified by a common, single amplifier. In order to amplify a pluralityof signals of different frequencies simultaneously, such amplifiersgenerally employ the feed forward construction which is suitable foreliminating cross modulation distortion.

In the conventional feed forward amplifiers, however, there has been aproblem that the equilibrium of the feed forward loop tend to be lostdue to a change in the operational environment such as ambienttemperature. Such a deviation of the feed forward loop results in adegradation in the compensation of distortions. In order to avoid such adeviation of the feed forward loop, therefore, the conventional feedforward amplifier generally has a control function for detecting theoperational state of the loop and for controlling the loop automaticallyin response to the detected state of the loop.

FIG. 1 shows the fundamental construction of a well known feed forwardamplifier. Referring to FIG. 1, it will be noted that the feed forwardamplifier is generally formed of a distortion extraction loop 1 and adistortion elimination loop 2.

In the distortion extraction loop 1, an input RF signal includingtherein a main signal component is supplied to an input terminal 3,wherein the RF signal is branched by a hybrid circuit 4 into a firstbranch signal and a second branch signal. The first branch signal isamplified in a main amplifier 5 and branched further in a directionalcoupler 6. One of the signals thus branched in the directional coupler 6is then supplied to a power synthesizer 8 via an attenuator 7. On theother hand, the signal of the second branch is supplied to a variableattenuator 10 and a variable phase shifter 11 via a delay line 9, andthe signal thus passed through the attenuator 10 and the phase shifter11 is supplied to the power synthesizer 8. Thereby, the variableattenuator 10 and the variable phase shifter 11 are set such that bothmain signal components, supplied to the power synthesizer 8 along thefirst and second paths, have an equal amplitude and delay time as wellas a mutually opposite phase difference of 180° (inverse phaserelationship). As a result, the main signal components are suppressed inthe output of the power synthesizer 8, and the output of the powersynthesizer 8 contains only the non-linear distortion components thatare formed in the main amplifier 5.

In the distortion elimination loop 2, on the other hand, the otheroutput of the directional coupler 6 is supplied to the power synthesizer13 via a delay line 12. Further, the non-linear distortion componentsextracted by the power synthesizer 8 are supplied to a secondaryamplifier 16 for amplification via a variable attenuator 14 and avariable phase shifter 15. After amplification in the secondaryamplifier 16, the signal is supplied to the power synthesizer 13. Inthis case, too, the variable attenuator 14 and the variable phaseshifter 15 are set such that both of the distortion components suppliedto the power synthesizer 13 via respective paths have the same amplitudeand delay time as well as a mutually opposite phase difference of 180°and thereby the distortion components are suppressed effectively fromthe output RF signal of the directional coupler 6 and, hence, in theoutput RF signal of the main amplifier 5. As a result, the distortion ofthe main signal is minimized.

As described above, the feed forward amplifier functions effectivelyonly when the two suppression loops are set properly. When thecharacteristics of various elements forming the loop have changed as aresult of the environmental change such as the change of ambienttemperature, on the other hand, the desired effect of the loop forsuppressing distortion reduces substantially.

In order to overcome this problem, a construction shown in FIG. 2 isproposed in the Japanese Laid-open Patent Publication 1-198809. In theconventional art disclosed therein, there is provided anotherdirectional coupler 18 on the output line of the main amplifier 5 incorrespondence to the distortion extraction loop 1, in addition to thefundamental construction of FIG. 1. Thereby, a pilot signal produced bya pilot signal generator 19 is injected to the output RF signal a thatis outputted from the main amplifier 5. Further, a directional coupler23 is provided on the output line of the secondary amplifier 16 which isincluded in the distortion elimination loop 2, for branching the outputof the amplifier 16. One of the output signals of the secondaryamplifier 16 thus branched in the directional coupler 23 is supplied toa power detector 24. Further, there is provided another directionalcoupler 20 on the output line of the power synthesizer 13, wherein anoutput signal c of the feed forward amplifier, which has been branchedin the directional coupler 20, is supplied to a pilot signal detector 21together with the pilot signal produced by the pilot oscillator 19.Further, the output signal of the pilot detector 21 and the output ofthe power detector 24 are supplied to a control circuit 22. The controlcircuit 22, in turn, supplies control voltage signals to the variableattenuators 10 and 14 and the variable phase shifters 11 and 15 in thedistortion extraction loop 1 and in the distortion elimination loop 2.

In the control circuit 22, the variable attenuator 10 and the variablephase shifter 11 are controlled such that the output voltage signal ofthe power detector 24 is minimized. In response to this, the degree ofsuppression of the main signal component in the output signal(distortion signal b) of the distortion extraction loop 1 is maximized.In the foregoing control operation, the total power of the main signal aand the distortion signal b is detected, and the variable phaseattenuator 10 and the variable phase shifter 11 are controlled such thatthe detected power is minimized. As a result, the suppression of themain signal component in the output distortion signal outputted from thedistortion extraction loop is maximized.

FIGS. 3(A)-3(H) show the operation of the pilot detector 21. The pilotdetector 21 is formed of a mixer 211 shown in FIG. 3(B) and a low passfilter 212 shown in FIG. 3(F), wherein the mixer 211 is supplied withmain signal components f₁ and f₂ and distortion signal components f₃ andf₄ shown in FIG. 3(A) and further with a pilot signal f_(o) ' shown inFIG. 3(C). Thereby, ten sets of frequency component signals shown inFIG. 3(D) are obtained in the output of the mixer 211. As the signalcomponent f_(p) ' and the signal component f_(p) have the samefrequency, the sum of the signal components f_(p) and f_(p) ' producedby the mixer 211 has a frequency of 2f_(p) ' (2f_(p)). Further, themixer 211 produces a d.c. component of zero frequency as the differencesignal.

Thus, by causing the output signal of the mixer 211 shown in FIG. 3(E)to pass through the low pass filter 212, the frequency components shownin FIG. 3(G) are eliminated, and the d.c. component shown in FIG. 3(H)is obtained as the difference signal having the frequency of f_(p)'-f_(p).

Meanwhile, the control circuit 22 controls the variable attenuator 14and the variable phase shifter 15 such that the d.c. output voltage ofthe pilot detector 21 shown in FIG. 3(H) becomes minimum. In otherwords, the control circuit 22 controls the variable attenuator 14 andthe variable phase shifter 15 such that the pilot signal component inthe output RF signal a and the pilot signal component in the distortionoutput signal b have a mutually identical level and mutually oppositephases. As a result of such a control, the elimination of distortioncomponents in the distortion elimination loop 2 is maximized. As thepilot signal injected to the output of the main amplifier 5 passesthrough the same signal path as the distortion components formed in themain amplifier 5, the suppression of the pilot signal is equivalent tothe suppression of the distortion components. FIGS.4(A)-4(C) show thewaveform of the signals a-c in the distortion elimination loop 2.

In such a conventional feed forward amplifiers, on the other hand, therehas been a problem in that a long time is needed for reaching aconvergence of operation of the feed forward loop because of the factthat the variable attenuator 10 and the variable phase shifter 11 in thedistortion elimination loop 1 are controlled in response to the outputof the pilot detector 21 while the variable attenuator 14 and thevariable phase shifter 15 are controlled simultaneously. In the worstcase, the operation of the feed forward amplifier may not converge atall.

The above problem will be described in more detail with reference toFIGS. 5(A)-5(C), wherein FIG. 5(A) shows the case where both themagnitude and phase of the output signal c change in response to achange in the phase of the distortion signal b. In FIG. 5(A), it shouldbe noted that there is no substantial change in the magnitude of thesignal b. FIG. 5(B), on the other hand, shows the case wherein both themagnitude and phase of the output signal c change in response to achange in the magnitude of the distortion signal b alone. In the case ofFIG. 5(B), the phase of the signal b does not change substantially.

Thus, in the conventional system, it is extremely difficult to obtainthe desired convergence of the system as shown in FIG. 5(C) by merelychanging the phase and the magnitude of the distortion signal b at thesame time.

In order to overcome the problem, the conventional system has employed acomplex time sequential process for controlling the foregoing loop 1 andthe loop 2 separately. However, such a time sequential process iscomplex and takes a long time until the operation of the loop converges.Further, such a system has a drawback in that one cannot determine whichone of the loop 1 and the loop 2 contains a defect when the system showsa failure.

Further, in the RF amplifiers for use in the base stations of mobiletelephone network, the input level tends to change with time in responseto the movement of the mobile terminals. Thereby, there may be a casewhere one cannot ascertain the cause of the apparent variation of theoutput voltage signal of the power detector 24. It will be noted thatone cannot ascertain whether such a variation has been caused as aresult of the problems in the feed forward loop or as result of thevariation in the level of the incoming RF signal. When the optimizationof the feed forward loop is carried out in such a situation, there caneven be a case where the cross distortion in the signal increases as aresult of the control.

In the feed forward amplifier of FIG. 2 and with reference to FIG. 6, itshould also be noted that the non-linear distortion components in the RFsignal B, which signal B being outputted from the amplifier 5, arecanceled out by the non-linear distortion components contained in the RFsignal E that is produced by the variable phase shifter 15, as indicatedschematically in FIG. 6. Thereby, the control of the variable attenuator10 or 14, as well as the control of the variable phase shifter 1 or 15,are achieved based upon the residual non-linear distortion componentsremaining in the RF signal F by detecting the residual non-lineardistortion components by the directional coupler 20. The non-lineardistortion component remaining in the signal F is naturally very small,as a result of the distortion compensation operation conducted by thefeed forward loop 1, and because of this, there is a tendency that thecontrol of the distortion extraction loop 1 or the distortionelimination loop becomes unstable. This problem is not resolvedcompletely even when a pilot signal is injected to the output of theamplifier 5 as in the circuit of FIG. 2. In FIG. 6, it should be notedthat the signal A represents the spectrum of the RF signal as suppliedto the feed forward amplifier, the signal B represents the spectrum ofthe RF signal outputted from the amplifier 5, the signal C representsthe spectrum of the RF signal outputted from the variable phase shifter11, the signal D represents the spectrum of the RF signal outputted fromthe power synthesizer 8, the signal E represents the spectrum of the RFsignal outputted from the variable phase shifter 15, and the signal Frepresents the spectrum of the output RF signal detected by thedirectional coupler 20. While the spectrum A contains only the mainsignal components, it will be noted that the spectrum B contains thenon-linear distortion components in addition to the main signalcomponents. Further, the spectrum C contains the main signal componentsin the state that the phase thereof is inverted, while the spectrum Drepresents the non-linear distortion components that have been extractedby canceling out the main signal components in the spectrum C by themain signal components in the spectrum B. Further, the spectrum Erepresents the signal components wherein the phase of the non-lineardistortion components is inverted.

DISCLOSURE OF THE INVENTION

Accordingly, it is a general object of the present invention to providea novel and useful feed forward amplifier.

Another and more specific object of the present invention is to providea feed forward amplifier including a distortion extraction loop forextracting non-linear distortion components formed in a main amplifierand a distortion elimination loop for canceling out said distortioncomponents by injecting said non-liner distortion components into anoutput signal of said main amplifier after modification, wherein saidfeed forward amplifier controls said two loops independently forreducing the time needed for achieving control, said feed forwardamplifier thereby facilitating a detection of failures, and wherein saidfeed forward amplifier operates with reliability even under existencewhen of variations in the level of the incoming input signals exist.

Another object of the present invention is to provide a feed forwardamplifier comprising:

RF amplification means supplied with an input RF signal at an inputterminal for amplifying the same, said RF amplification means therebyproducing an output RF signal;

a distortion extraction loop supplied with said input RF signal fromsaid input terminal and further with said output RF signal from said RFamplification means, for extracting non-linear distortion componentsformed in said output RF signal as a result of amplification in said RFamplification means;

variable phase shifter means provided in said distortion extraction loopfor varying a phase of said input RF signal that has been supplied tosaid distortion extraction loop;

variable attenuation means provided in said distortion extraction loopfor attenuating an amplitude of said input RF signal that has beensupplied to said distortion extraction loop; and

distortion extraction means provided in said distortion extraction loop,said distortion extraction means being supplied on the one hand withsaid input RF signal after processing in said variable phase shiftermeans and said variable attenuation means and on the other and with saidoutput RF signal from said RF amplification means, for producing adistortion output signal that includes non-linear components;

wherein said feed forward amplifier further comprises control meanssupplied with said input signal that has been supplied to said inputterminal and further with said distortion output signal for extracting amain signal component contained in said distortion output signal, saidcontrol means controlling said variable phase shifter means and saidvariable attenuation means such that a ratio in level of said mainsignal component is decreased with respect to said input RF signal.

According to the present invention, one can minimize the level of themain signal component in the distortion output signal relatively to thelevel of the input RF signal. Associated therewith, the suppression ofthe main signal component in the foregoing distortion output signal ismaximized even when the power level of the incoming RF signal has beenchanged. Thus, the feed forward amplifier of the present invention isparticularly suitable for the broad band RF amplifiers used in the basestation of mobile telephone networks wherein the level of the input RFsignal changes variously with time. In the feed forward amplifier of thepresent invention, as the suppression of the main signal component inthe distortion output signal is achieved with respect to the level ofthe input RF signal, the operation of the amplifier converges easily bya simple control system. Further, because of the fact that theoptimization of the distortion extraction loop and the optimization ofthe distortion elimination loop are carried out independently from eachother, the optimization process of the distortion extraction loop doesnot cause interference with the optimization process of the distortionelimination loop, and the control system is quickly stabilized to aconverged state. Further, according to the present invention, anyabnormal operation of the distortion extraction loop is easily detectedby examining whether or not the level of the main signal component inthe distortion output signal exceeds a predetermined threshold.

Another object of the present invention is to provide a feed forwardamplifier comprising:

RF amplification means supplied with an input RF signal at an inputterminal for amplifying the same, said RF amplification means therebyproducing an output RF signal;

a distortion extraction loop supplied with said input RF signal fromsaid input terminal and further with said output RF signal from said RFamplification means, for extracting non-linear distortion componentsformed in said output RF signal as a result of amplification in said RFamplification means;

distortion elimination means supplied with a distortion output signalfrom said distortion extraction loop and further with said output RFsignal from said amplification means, for canceling out said non-linerdistortion components contained in said output RF signal by saidnon-linear distortion components contained in said distortion outputsignal;

pilot signal generation means for producing a pilot signal; and

pilot signal injection means for injecting said pilot signal to saidoutput RF signal produced by said RF amplification means;

wherein said feed forward amplifier further comprises:

pilot signal detection means supplied with said pilot signal from saidpilot signal generation means for detecting a pilot signal componentcontained in a distortion-eliminated output signal that has beenproduced by said distortion elimination means, said pilot signaldetection means conducting a d.c. detection by mixing said pilot signalinto said distortion output signal; and

control means supplied with a d.c. detection signal from said pilotsignal detection means, said d.c. detection signal being obtained insaid pilot signal detection means as a result of said d.c. detection,said control means controlling said distortion elimination means suchthat said d.c. detection signal is minimized.

According to the present invention, the pilot signal component containedin the distortion-eliminated output signal is subjected to the d.c.detection. Thereby, the main signal component contained in thedistortion output signal is detected with reliability. Further, the mainsignal component contained in the foregoing distortion output signal isminimized with reliability. As the pilot signal is injected into theoutput signal of the foregoing RF amplification means, the signal pathof the pilot signal is identical with the signal path of the non-lineardistortion components. Thereby, the pilot signal behaves exactly likethe non-linear distortion components.

Another object of the present invention is to provide a feed forwardamplifier having a distortion extraction loop, wherein the pilot signal,which has been injected to an output RF signal produced by an amplifierin the distortion extraction loop for detection of non-lineardistortion, can be detected with a high precision even after the processfor eliminating said non-linear distortion has been conducted in adistortion elimination loop.

Another object of the present invention is to provide a feed forwardamplifier comprising:

RF amplification means supplied with an input RF signal for producingthe same to form an output RF signal;

a distortion extraction loop supplied with said input RF signal andfurther with said output RF signal from said RF amplifier for extractingnon-linear distortion components formed in said output RF signal as aresult of amplification in said RF amplifier, said distortion extractionloop thereby producing a distortion output signal that contains thereinnon-linear distortion components; and

a distortion elimination loop supplied with said distortion outputsignal from said distortion extraction loop and further with said outputRF signal from said amplification means for canceling out saidnon-linear distortion components contained in said output RF signal bynon-linear distortion components contained in said distortion outputsignal;

said distortion elimination loop comprising:

variable attenuation means supplied with said distortion output signalobtained by said distortion extraction means for modifying an amplitudeof said distortion output signal;

variable phase shifter means supplied with said distortion output signalobtained by said distortion extraction means for varying a phase of saiddistortion output signal; and

distortion elimination means supplied with said distortion output signalafter processing in said variable attenuation means and said variablephase shifter means, said distortion elimination means being furthersupplied with said output RF signal from said amplification means forsynthesizing said distortion output signal and said output RF signal,said distortion elimination means thereby canceling out the non-lineardistortion components contained in said output RF signal by thenon-linear distortion components contained in said distortion outputsignal;

wherein said feed forward amplifier further comprises control meanssupplied with said distortion output signal and further with said outputRF signal for controlling said variable attenuation means and saidvariable phase shifter means, such that said non-linear distortioncomponents in said output RF signal have an amplitude and a phase thatare identical to an amplitude and a phase of said non-linear distortioncomponents contained in said distortion output signal.

According to the present invention, the control of the distortionelimination loop is achieved based upon the output RF signal produced bythe RF amplification means and further upon the distortion output signalproduced by the distortion extraction loop. Thereby, the problempertinent to the conventional feed forward amplifiers, that the controlof the distortion elimination loop is carried out based upon the minute,residual non-linear distortion components that remain after thecancellation of distortion, is effectively avoided, and a reliableoptimization of the distortion elimination loop is achieved.

Another object of the present invention is to provide a feed forwardamplifier wherein an effective compensation of non-linear distortion isachieved by a simple control process.

Another object of the present invention is to provide a feed forwardamplifier comprising:

RF amplification means supplied with input RF signal for amplifying thesame to produce an output RF signal;

a distortion extraction loop supplied with said input RF signal andfurther with said output RF signal of said RF amplification means forextracting non-linear distortion components formed in said output RFsignal as a result of amplification by said RF amplifier, saiddistortion extraction loop thereby producing a distortion output signalthat contains said non-liner distortion components; and

a distortion elimination loop supplied with said distortion outputsignal from said distortion extraction loop and further with said outputRF signal from said RF amplification means, for canceling out saidnon-linear distortion components contained in said output RF signal bysaid non-linear distortion components contained in said distortionoutput signal;

wherein said distortion extraction loop comprises:

variable phase shifter means supplied with said input RF signal forchanging a phase thereof;

phase shifter means supplied with said output RF signal from said RFamplification means for causing a 90° degree phase shift therein;

synchronous detection means supplied with said input RF signal afterprocessing in said variable phase shifter means and further with saidoutput RF signal processed by said phase shifter means for carrying outa synchronous detection, said synchronous detection means therebydetecting main signal components contained in said output RF signal;

control means supplied with said main signal components detected by saidsynchronous detection means for controlling said variable phase shiftermeans such that said main signal components are minimized; and

distortion extraction means supplied with said input RF signal afterprocessing by said variable phase shifter means and further with saidoutput RF signal produced by said RF amplification means, for causing acancellation of said main signal contained in said input RF signal andmain signal components contained in said output RF signal, saiddistortion extraction means producing an output signal essentiallyconsisting of said non-linear distortion components, as an outputsignal.

According to the present invention, the variable phase shifter means iscontrolled explicitly based upon the main signal components outputtedfrom the synchronous detection means, such that the main signalcomponents in the input RF signal that has been supplied to theforegoing distortion extraction means have a phase shifted by 180° withrespect to the phase of the main signal components in the output RFsignal. As a result, the extraction of distortion components is achievedby a simple control process. Further, one can eliminate the injection ofthe pilot signal for extracting the distortions.

Another object of the present invention is to provide a feed forwardamplifier wherein the elimination of distortion can be achieved more orless completely.

Another object of the present invention is to provide a feed forwardamplifier comprising:

RF amplification means supplied with an input RF signal from an inputterminal for amplifying the same to produce an output RF signal;

a plurality of distortion extraction loops each being supplied with saidinput RF signal from said input terminal and further with said RF signalfrom said RF amplification means for extracting non-linear distortioncomponents formed in said output RF signal as a result of amplificationby said RF amplification means, each of said distortion extraction loopsproducing a distortion output signal containing therein said non-lineardistortion components;

a plurality of distortion extraction loops provided in correspondence tosaid plurality of distortion extraction loops, said distortionextraction loops being cascaded with each other and supplied with saidoutput RF signal from said RF amplification means, each of saiddistortion extraction loops being further supplied with a correspondingdistortion output signal from a corresponding distortion extraction loopfor removing said non-linear distortion components from said output RFsignal supplied thereto; and

distortion elimination loop control means provided in each of saidplurality of distortion elimination loops, each of said distortionelimination loop control means being supplied with said output RF signalfrom which said non-linear distortion components are eliminated, saiddistortion elimination loop control means controlling said correspondingdistortion elimination loop such that said non-linear distortioncomponents remaining in said output RF signal is minimized.

According to the present invention, the process of elimination ofdistortion is achieved for a plurality of times, and a completeelimination of distortion is achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the construction of a conventional,fundamental feed forward amplifier;

FIG. 2 is a diagram showing the construction of a conventional feedforward amplifier having a control system that operates based upon apilot signal;

FIGS. 3(A)-3(H) are diagrams showing the operation of the feed forwardamplifier of FIG. 2;

FIGS. 4(A)-4(C) are other diagrams showing the operation of the feedforward amplifier of FIG. 2;

FIGS. 5(A)-5(C) are diagrams showing the principle of cancellation ofdistortion in the feed forward amplifier of FIG. 2;

FIG. 6 is a diagram for explaining another problem occurring in theconventional feed forward amplifier;

FIG. 7 is a diagram showing the construction of a feed forward amplifieraccording to a first embodiment of the present invention;

FIG. 8 is a diagram showing the construction of the power detector usedin the circuit of FIG. 7;

FIG. 9 is a diagram showing the construction of a feed forward amplifieraccording to a second embodiment of the present invention;

FIG. 10 is a flowchart showing the overall control sequence of the feedforward amplifier of FIGS. 8 and 9;

FIG. 11 is a flowchart showing the control sequence of a distortionextraction loop shown in FIG. 10;

FIG. 12 is a flowchart showing the control sequence of the distortionelimination loop of FIG. 10;

FIG. 13 is a diagram showing the construction of a feed forwardamplifier according to a third embodiment of the present invention;

FIGS. 14(A)-14(D) are diagrams showing the principle of the feed forwardamplifier of FIG. 13;

FIG. 15 is another diagram showing the principle of the feed forwardamplifier of FIG. 13;

FIG. 16 is a flowchart showing the overall control sequence of the feedforward amplifier of FIG. 13;

FIG. 17 is a diagram showing the construction of a feed forwardamplifier according to a fourth embodiment of the present invention;

FIG. 18 is a diagram for explaining the operation of the feed forwardamplifier of FIG. 17;

FIG. 19 is a diagram showing the construction of a feed forwardamplifier according to a fifth embodiment of the present invention;

FIG. 20 is a diagram for explaining the operation of the feed forwardamplifier of FIG. 19;

FIG. 21 is a diagram showing the construction of a feed forwardamplifier according to a sixth embodiment of the present invention;

FIG. 22 is a diagram showing the construction of a part of the feedforward amplifier of FIG. 21;

FIG. 23 is a diagram showing the construction of a feed forwardamplifier according to a seventh embodiment of the present invention;

FIG. 24 is a diagram showing the construction of a feed forwardamplifier according to an eighth embodiment of the present invention;

FIG. 25 is a diagram showing the construction of a feed forwardamplifier according to a ninth embodiment of the present invention;

FIG. 26 is a diagram showing the construction of the feed forwardamplifier according to a tenth embodiment of the present invention; and

FIG. 27 is a diagram showing a feed forward amplifier according to aneleventh embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 7 is a block diagram showing the construction of a feed forwardamplifier according to a first embodiment of the present invention. InFIG. 7, those parts corresponding to the parts described previously withreference to FIGS. 1 and 2 are designated by the same reference numeralsand the description thereof will be omitted.

In the present embodiment, a directional coupler 25 is provided betweenthe input terminal 3 and the hybrid circuit 4 for branching a part ofthe input RF signal supplied to the input terminal 3. The RF signalbranched at the coupler 25 is then supplied to an input power detector26, wherein the input power detector 26 detects the power of the inputRF signal supplied thereto as the power level of the incoming input RFsignal. Further, the input power detector 26 produces an output signalVid indicative of the power of the input RF signal and supplies the sameto a first control unit 22A forming a part of the control circuit 22.

In the circuit of the present embodiment, there is provided adirectional coupler 27 on the output path of the power synthesizer 8,wherein the directional coupler 27 branches away a part of the outputdistortion signal that contains therein the non-linear distortioncomponents and supplies the branched output signal to a main signaldetector 28. The main signal detector 28 detects the power of the mainsignal components that are contained in the distortion output signalsupplied thereto and produces an output signal Vcd indicative of thedetected power of the main signal components. The output signal Vcd isthen supplied to the foregoing first control unit 22A. The control unit22A in turn controls the variable attenuator 10 and the variable phaseshifter 11 based upon the signals Vid and Vcd supplied thereto such thatthe signal Vcd has a level smaller than the level of the signal Vid by apredetermined factor such as 20 dB or more.

As a consequence of the construction as such, the circuit of FIG. 7 doesnot use the directional coupler 23 for branching the output signal ofthe secondary amplifier 16 or the power detector that detects the mainsignal components from the output of the directional coupler 23. Inother words, the construction of FIG. 7 achieves the extraction of thenon-linear distortion. components in the distortion extraction loop 1such that the proportion of the main signal components that arecontained in the distortion output signal becomes minimum relative tothe main signal components that are contained in the input RF signalsupplied to the input terminal 3. Thereby, the extraction of thenon-linear distortion is achieved with reliability in the distortionextraction loop 1 even in the case where the transmitted RF powerchanges with time in response to the movement of mobile terminals as inthe case of the base station of mobile telephone network.

FIG. 8 shows the construction of power detectors 26 and 28 used in thecircuit of FIG. 7. In the illustrated example, the circuit has a usualconstruction wherein the input RF signal supplied to the input terminalIN is detected by a diode D and smoothed subsequently by a resistor Rand a capacitor C. In other words, the power detectors 26 and 28 outputa voltage signal indicative of an average power averaged over the entirespectrum of the supplied RF signal.

In the embodiment of FIG. 7, it should be noted that the constructionand operation of other parts of the circuit are substantially identicalwith those of FIG. 2. Thus, the pilot signal produced by the pilotsignal generator 19 is injected into the output of the main amplifier 5at the directional coupler 18, wherein the pilot signal thus injectedpasses through the directional coupler 6 together with the non-lineardistortion components formed by the amplifier 5. The pilot signal isthereafter divided into a first path that includes the delay line 12 anda second path that includes the attenuator 7 and the power synthesizer8. The main signal components are canceled out in the power synthesizer8 as already explained, and as a result, the non-linear distortioncomponents and the pilot signal are extracted. The non-linear distortioncomponents and the pilot signal thus extracted are supplied to thedirectional coupler 13 after passing through the variable attenuator 14,the variable phase shifter 15 and the secondary amplifier 16. Thereby,the main signal components are extracted by the directional coupler 13from the output RF signal by canceling out the non-linear distortioncomponents from the output RF signal by the distortion output signalthat has been supplied to the directional coupler 13.

The main signal components thus extracted in the directional coupler 13are on the one hand supplied to an output terminal 17 via thedirectional coupler 20, wherein the directional coupler 20 branches apart of the main signal component and supplies the same to the pilotdetector 21. The pilot detector 21 in turn detects the pilot signal thatis remaining in the output signal of the directional coupler 13 togetherwith the non-linear distortion components, and supplies the detectedsignal to a second control unit 22B that forms a part of the controlcircuit. The second control unit 22B in turn controls the variableattenuator 14 and the variable phase shifter 15 in response to theoutput of the pilot signal detector 13 such that the pilot signal andthe non-linear signal component remaining in the output of thedirectional coupler 13 is minimized.

Next, a second embodiment of the present invention will be describedwith reference to FIG. 9.

In the present embodiment, another variable attenuator 33 is providedbetween the directional coupler 25 and the distortion extraction loop 1,in addition to the construction of FIG. 7. Further, an attenuator 30 anda power detector 31 are provided for detecting the output power of thefeed forward amplifier, based upon the output signal branched by thedirectional coupler 20. It should be noted that the power detector 31detects the level of the output signal of the attenuator 30. Further,there is provided an AGC circuit 32 that controls the variableattenuator 33 in response to the output power detected by the powerdetector 31 and, further in response to the input power that has beendetected by the input power detector 26.

In operation, the AGC amplifier 32 controls the variable attenuator 33such that the difference between the input power that has been detectedby the detector 26 and the output power that has been detected by thedetector 31 becomes zero. In this construction, it should be noted thatthe gain of the feed forward amplifier becomes identical with theattenuation of the attenuator 30, provided that the power detector 26and the power detector 31 have the same construction. As the outputpower level of the .feed forward amplifier is reduced by a magnitudecorresponding to the attenuation of the attenuator 30, the adjustment ofthe variable attenuator 33 for setting the difference of the power ofthe input signals to be zero at the AGC amplifier 32 results in aconstant difference between the input and output powers of the feedforward amplifier. Since the output power is detected relatively withrespect to the input power, the feed forward amplifier maintains aproper operation even when the input power level variably changes.

The foraging automatic control of the feed forward amplifier is achievedin order to compensate for the temperature variation in various parts ofthe system. Naturally, the gain of the main amplifier 5 changes with thevariation of the temperature. The larger the gain of the main amplifier5, the larger the effect of the temperature variation. Thus, the feedforward amplifier of the foregoing embodiment can not only compensatefor the distortion but also provide a stabilized control of the gain.

FIGS. 10-12 show the algorithms of the control circuit 22A shown in FIG.7 and FIG. 9, wherein FIG. 10 shows the flowchart for the overalloperation of the control circuit 22, FIG. 11 shows the flowchart for theoperation of the variable phase shifter (PSV) 11 shown in the flowchartof FIG. 10 and FIG. 12 shows the flowchart of the operation of thevariable attenuator (VATT) 10, shown also in FIG. 10.

Referring to FIG. 10, an index M indicative of the number of iterationsis set to "0" (Step T1) and the control of the variable phase shifter(PSV) 11 is carried out subsequently (Step T2). Further, the control ofthe variable attenuator (VATT) 10 is carried out subsequently (Step T3).After this, the index M is increased by "1" (Step T4), and the stepsT2-T5 are repeated (Step T5) until the index M exceeds a predeterminedvalue x.

The step T2 shown in FIG. 10 is carried out in accordance with theflowchart shown in FIG. 11. First, the control circuit 22A reads theoutput voltage Vid of the input power detector 26 and the output voltageVcd of the main signal detector 28 (Step S1), and calculates the ratioVc=Vcd/Vid (Step S2). In this example, it should be noted that the powerdetectors produce the output signals in the form of voltage signals.

Next, the initial direction for the control of the variable phaseshifter 11 is set (step S3), and the variable phase shifter 11 is drivenby one step (Step S4). In this state, the output voltages Vid and Vcdare read again (Step S5), and the ratio Vc=Vcd/Vid is calculated again(Step S6). Further, a comparison is made (Step S7) for the parameter Vcwith respect to the value of the parameter Vc before the control is made(before the Step S4), and a discrimination is made (Step S8) whether theparameter Vc has decreased as compared to the parameter Vc before thecontrol. If the value of the parameter Vc has not decreased, thisindicates that the direction of control is opposite. Thus, the state ofthe variable phase shifter 11 is stepped back by one (Step S9) and thedirection of the control is set opposite (Step S10). Further, the stepsS4-S10 are repeated until the value of the ratio Vc becomes smaller thanthe value of the parameter (ratio) Vc before the control is made.

When it is confirmed in the step S8 that the value of the ratio Vc hasdecreased before the control has been made, the variable phase shifter11 is driven by one step further (Step S1) and the output voltages Vidand Vcd are read again (Step S12). Further, the ratio Vc=Vd/Vid iscalculated (Step S13) and the value of the ratio Vc thus obtained iscompared (Step S14) with the ratio Vc before the control (Step S11).Further, a discrimination is made whether or not the value of the ratioVc has decreased as compared with the ratio Vc before the control ismade (Step S15). When the ratio Vc has decreased, this indicates thatthere is still room for adjustment and the process returns to the stepS11 for repeating the steps S12-S14. On the other hand, when it isconfirmed as a result of the step S15 that the value of the ratio Vcdoes not decrease anymore, the state of the variable phase shifter 11 isstepped back by one (Step S16), and the voltages Vid and Vd are read("Read") again (Step S17). Further, the ratio Vc=Vcd/Vid is calculatedagain (Step S18) and the process returns to the flowchart of FIG. 10.

On the other hand, the step T3 shown in FIG. 10 is carried out accordingto the flowchart of FIG. 12, wherein it will be noted that the processshown in the flowchart of FIG. 12 is substantially identical with theprocess shown in the flowchart of FIG. 11 except for the fact that thevariable attenuator (VATT) 10 is used in place of the variable phaseshifter (PSV) 11. For example, the steps S3-S18 of FIG. 11 correspond tothe steps S21-S38 of FIG. 12. Thus, description of FIG. 12 will beomitted.

It should be noted that the control sequence of FIGS. 11 and 12represents a process for seeking the minimum of the ratio Vcd/Vid. Onthe other hand, one may detect an event that the ratio Vcd/Vid hasdecreased below a predetermined threshold such as 20 dB and interruptthe control in response to such an event. In such a case, the ratioVcd/Vid is subjected to a discrimination process whether or not theratio Vcd/Vid has reduced to the foregoing predetermined value in thestep S8 or Step S28, and if the result is NO, the process returns to thestep S4 or the step S24. On the other hand, when the result is YES, theprocess returns to the flowchart of FIG. 10.

In each of the embodiments of FIG. 7 and FIG. 9, it should be noted thatthe control of the distortion extraction loop 1 and the control of thedistortion elimination loop are achieved independently from each otherby separate control units, 22A and 22B. As a result, it is possible tooptimize the distortion elimination loop 2 after the distortionextraction loop 1 has been optimized. Thereby, the problem of theoptimization of the loop 1 and the optimization of the loop 2 cause aninterference is successfully avoided. Associated therewith, the problemof a prolonged operational duration needed for achieving the convergenceof the system is avoided.

Next, a third embodiment of the present invention will be described withreference to FIG. 13. In FIG. 13, those parts described previously aredesignated by the same reference numerals.

The present embodiment represents an improvement of the first embodimentshown in FIG. 7 and hence includes a distortion extraction loop 1 and adistortion elimination loop 2 similar to the loops described before. Asthe construction of the distortion extraction loop 1 is exactlyidentical with that of the first embodiment, further description will beomitted.

In the present embodiment, a d.c. detector that carries out a d.c.detection is used as the pilot signal detector 21 for detecting thepilot signal from the output RF signal that has been branched by thedirectional coupler 20. The d.c. detector 21 includes a variable phaseshifter 31 that is supplied with the pilot signal from the pilot signalgenerator 19 for controlling the phase thereof, a mixer 211 for mixingthe pilot signal processed by the variable phase shifter 31 and theoutput RF signal that has been branched by the directional coupler 20,and a low pass filter 212 for filtering the output of the mixer 211,wherein the mixer 211 and the filter 212 produce an output signal e andsupplies the same to a control unit 41 and further to a control unit 42,both thereby a part of the control unit 22B. The control unit 41 in turncontrols the variable phase shifter 31 based upon the detected pilotsignal obtained by the detector 21 such that the phase of the pilotsignal supplied to the mixer 211 and the phase of the pilot signalcontained in the output RF signal become identical with each other. As aresult, the output signal e of the filter 212 represents the d.c.voltage that represents the level of the detected pilot signal.

Further, the foregoing control unit 42 is supplied with the d.c. voltagesignal e from the filter 212 and controls the variable attenuator 14 andthe variable phase shifter 15 such that the magnitude of the foregoingd.c. voltage signal e is minimized. As a result, the pilot signal aswell as the non-liner distortion components remaining in the output RFsignal obtained by the directional coupler 13 are minimized, and oneobtains an output RF signal from which the non-liner distortioncomponents are eliminated at the output terminal 17.

FIGS. 14(A)-14(D) are the diagrams showing the operational principle ofthe pilot signal detector 21, wherein FIG. 14(A) shows the constructionof the pilot signal detector 21 formed of the mixer 211 and the low passfilter 212. It will be noted that the mixer 211 is supplied with thepilot signal from the variable phase shifter 31 and further with theoutput RF signal from the directional coupler 20. FIG. 14(B) shows thespectrum of the output RF signal supplied to the mixer 211, wherein itwill be noted that the output RF signal contains residual non-lineardistortion components f₃ and f₄ and a residual pilot signal f_(p) ' inaddition to the main signal components f₁ and f₂. FIG. 14(C), on theother hand, shows the spectrum of the pilot signal. As the signal ofFIG. 14(C) is the pilot signal itself, the FIG. 14(C) contains only thepilot signal component f_(p) '. FIG. 14(D) on the other hand shows thespectrum of the output signal obtained from the filter 212 in thedetector 21. As can be seen in FIG. 14(D), the detector 21 produces thed.c. voltage signal e as a result of the mixing of the output RF signaland the pilot signal followed by the low-pass filtering.

FIG. 15 shows the variation of the output voltage e of the detector 21that occurs when the phase of the output RF signal and the phase of thepilot signal component in the pilot signal have changed relatively witheach other. As can be seen in FIG. 15, the output d.c. voltage e becomesmaximum when the phases of the respective pilot signal components inboth signals coincides with each other. On the other hand, the outputd.c. voltage e becomes zero when the phase difference between therespective plot signals has become 90°.

In the present embodiment, as shown in the flowchart of FIG. 16, thephase shifter 31 is controlled in a step S1 such that the voltage ebecomes maximum. Of course, the control is achieved by the control unit41 in the control unit 22B as described before. After the voltage e isthus maximized in the step S1, a step S2 is carried out wherein thecontrol unit 42 in the control unit 22B is activated to control thevariable attenuator 14 and the variable phase shifter 15 such that thevoltage e is minimized. It should be noted that the voltage e representsthe level of the pilot signal in the output RF signal and simultaneouslythe magnitude of the non-linear distortion components contained therein.By minimizing the voltage e, one can minimize the residual non-lineardistortion components remaining in the output RF signal. As the controlstep S1 and the control step S2 can be carried out independently, theoptimization in each of the steps does not cause interference to theoptimization in the other of the steps, and the optimization of thecontrol system converges quickly.

FIG. 17 shows a fourth embodiment of the present invention. In FIG. 17,those parts corresponding to the parts of the third embodiment aredesignated by the same reference numerals and the description thereofwill be omitted.

In the present embodiment, whereas the pilot detector 21 is formed ofthe mixer 211 and the low pass filter 212, similarly to the embodimentof FIG. 7, it will be noted that there is provided another pilotdetector 32. This pilot detector 32 includes a band pass filter 321 forextracting the pilot signal component from the signal c that has beenbranched by the directional coupler 20 and a variable attenuator 322 foradjusting the level of the output signal of the band pass filter 321, aswell as an amplifier 323 for amplifying the output signal of thevariable attenuator 322. Further, there is provided a detector 324 fordetecting the output of the amplifier 323 by conducting a d.c. detectionprocess, a control unit 325 for controlling the variable attenuator 322in response to the output signal of the detector 324, and a mixer 326supplied with an output signal i of the amplifier 323 and a part of theoutput signal j of the variable phase shifter 31 (this may be the outputsignal d itself of the variable phase shifter 31) for producing a d.c.detection signal f (hereinafter it is assumed that the mixer 326includes a low pass filter not illustrated). The control unit 42controls only the variable attenuator 14 and the variable phase shifter15 in response to the output signal e of the low pass filter 212.

In the present embodiment, the pilot signal remaining unremoved in thesignal c, even after the foregoing cancellation process of thedistortion components from the main signal a by the distortion signal b,is extracted by the band pass filter 321 and the signal thus extractedis supplied to an AGC circuit that is formed of the variable attenuator322, the amplifier 323, the detector 324 and the control unit 325. TheAGC circuit produces the signal i with a constant predetermined level(corresponding to the level of the original pilot signal of whichmagnitude is known), and the signal i thus produced is supplied to themixer 326.

The signal i and the signal j produced by the variable attenuator 31,are supplied to the mixer 326, while the mixer 326 produces a d.c.voltage component as the output signal f. Thereby, a control unit 33controls the variable phase shifter 31 so as to maximize the signal f,and the signal i and the signal are adjusted to j have the same phase(see FIG. 4(B)).

Thereby, the pilot signals c and d supplied to the mixer 211 areadjusted to have the same phase, and the control circuit 22A controlsthe variable attenuator 14 and the variable phase shifter 15 such thatthe voltage e thus obtained by the mixer 211 and the low pass filter 212is minimized. Ultimately, the distortion signal b is removed from themain signal a and one obtains an output signal g that is free from thedistortion. It should be noted that the foregoing signal d representsthe pilot signal branched from the signal j.

FIG. 18 shows a flowchart of the operation of the feed forward amplifierof FIG. 17.

Referring to FIG. 18, the pilot detector 32 detects the pilot signalcomponent contained in the output signal c corresponding to the outputRF signal of the feed forward amplifier c, and obtains the voltagesignal f as a result of the d.c. detection carried out with respect tothe pilot signal j, of which the phase is controlled by the variablephase shifter (EPS) 31 (Step S11).

Next, the phase control unit 33 controls the variable phase shifter 31in response to the voltage signal f from the pilot detector 32 such thatthe signal f is maximized (Step S12). Thereby, the phase of the signal jof the variable phase shifter 31 is set identical with the phase of thepilot signal component contained in the output signal c.

Next, the d.c. voltage e is extracted from the signal c and the signal dthat has been branched from the signal j (Step S13).

Further, the control circuit 22A controls the variable attenuator 14 andthe variable phase shifter 15 such that the d.c. voltage e from thepilot detector 21 is minimized similarly as before. Thereby, the voltagefor the pilot signal component (and hence the distortion) in thedistortion elimination loop 2 is minimized.

In the present embodiment, too, the control of each of the steps isachieved independently, and the problem such as slow convergence ofoperation does not occur.

Next, a feed forward amplifier according to a fifth embodiment will bedescribed with reference to FIG. 19. In FIG. 19, those parts describedalready are designated by the same reference numerals and thedescription thereof will be omitted.

Referring to FIG. 19, the present embodiment differs from the embodimentof FIG. 17 in the point that a mixer 327 and a low pass filter 328 areprovided between the amplifier 323 and the detector 324 forming thepilot detector 32, wherein the mixer 327 is supplied with the outputsignal i of the amplifier 323 and a pilot signal k that has beenbranched by a hybrid circuit 43, while the low pass filter 328 suppliesthe output signal of the mixer 327 to the detector 324.

In contrast to the case of FIG. 17 wherein the d.c. detection is carriedout by applying an AGC to the pilot signal, the level of which is knownbeforehand, the present embodiment applies the d.c. detection to thepilot signal after setting the level of the pilot signal in the outputRF signal c to be coincident to the actual level of the pilot signal.

More specifically, when setting the level of the pilot signal componentthat has been extracted by the band pass filter 321 to be coincident tothe predetermined level in the pilot detector 32, the signal i of theamplifier 323 and the foregoing pilot signal k are supplied to the mixer327 for the d.c. detection, wherein the d.c. component of the pilotsignal thus detected by the mixer 327 is supplied to the control unit325 via the low pass filter 328 and the detector 324. Thereby, thecontrol unit 325 achieves the automatic gain control by controlling thevariable attenuator 322 such that the signal i that is supplied to themixer 326 of the next stage and the signal j outputted from the variablephase shifter 321 have the same level. Thus, one can set both pilotsignal components to have the same phase and same level, by merelycontrolling the variable phase shifter 31 via the phase control unit 33,such that the voltage signal f outputted from the mixer 326 becomesmaximum.

In other words, by controlling the variable attenuator 14 and thevariable phase shifter 15 via the control circuit 22A such that thelevel of the signal e, which is obtained from the low pass filter 212 byfeeding the signal c and the signal d (which is equivalent to the signalj) to the mixer 211, to become minimum, one can eliminate the distortioncomponents from the main signal components.

Next, the operation of the feed forward amplifier of FIG. 19 will bedescribed with reference to the flowchart of FIG. 20.

First, the pilot detector 32 is activated to detect the pilot signalcomponent contained in the output signal c of the distortion eliminationloop 2 and sets the AGC circuit 32 such that the detected voltage isequal to the voltage of the original pilot signal. Further, the pilotsignal component thus detected and the pilot signal d, the phase ofwhich is subjected to the control by the variable phase shifter 31, aresupplied to the mixer 326 for the d.c. detection. As a result, oneobtains the d.c. voltage signal f (Step S21).

In response to the d.c. voltage signal f thus obtained, the phasecontrol unit 33 controls the variable phase shifter 31 such that thed.c. voltage signal f has the maximum level (Step S22). As a result ofthis process, the pilot signal component in the output signal c of thedistortion elimination loop 2 and the original pilot signal d are set tohave the same phase.

Based upon the pilot signals c and d set as such, the pilot detector 21extracts the information indicative of the d.c. voltage e (Step S23).

Thereafter, the control circuit 22A controls the variable attenuator 14and the variable phase shifter 15 such the d.c. voltage e of the pilotdetector 21 is minimized. In correspondence to this, the pilot signalcomponent (and hence the distortion components) in the distortionelimination loop 2 is minimized.

According to the present invention, the pilot signal component in thedistortion elimination loop 2 is set to have the same phase (and level)as the original pilot signal initially, and the cancellation of thepilot signal component is achieved thereafter by inverting the phase ofthe pilot signal components.

Next, a feed forward amplifier according to a sixth embodiment of thepresent invention will be described with reference to FIG. 21. In FIG.21, those parts corresponding to the previous embodiments are designatedby the same reference numerals and the description thereof will beomitted.

The present embodiment is an improvement of the conventional feedforward amplifier of FIG. 6 and includes the directional coupler 27 forbranching the distortion output signal outputted from the distortionextraction loop, the detector 28 for detecting the main signal componentremaining in the distortion output signal that has been branched by thedirectional coupler 27, and the control circuit 22A that minimizes themain signal component in the distortion output signal by controlling thevariable attenuator 10 and the variable phase shifter 11 in response tothe output of the detector 28.

In addition, in order to eliminate the problem of reliability ofdetection of the pilot signal as discussed with reference to FIG. 6, thepresent embodiment (FIGS. 21 and 22) includes a directional coupler 51in the distortion elimination loop 2 for branching the output RF signalthat has passed through the delay line 12, and a phase inverter 52 isprovided for inverting the phase of the output RF signal branched by thedirectional coupler 51. The phase inverter 52 produces an output signaldesignated as A, wherein there is provided a detector 53 for detectingthe signal A.

Further, the feed forward amplifier includes the directional coupler 23for branching the distortion output signal that has been processed bythe variable attenuator 14 and the variable phase shifter 15, and thereis further provided a detector 54 for detecting the pilot signal B thatis contained in the distortion output signal branched by the directionalcoupler 23. It will be noted that the output of the detector 43 and theoutput of the detector 54 are supplied to the control unit 55. Thecontrol unit 55 in turn controls the variable attenuator 14 and thevariable phase shifter 15 in response to the output signals of thedetectors 53 and 54, such that the signal A and the signal B have thesame amplitude and phase. In other words, the control unit 55 controlsthe signals A and B so that the relationships |A+B|=|A|+|B| and |A|=|B|hold.

As the signal A has a phase offset by 180° with respect to the signalthat is supplied to the directional coupler 13 from the directionalcoupler 51, one can cancel out the non-linear distortion componentscontained in the output RF signal that has passed through the delay line12 in the directional coupler 13 by the distortion output signalsupplied from the directional coupler 23. It should be noted thatnon-linear distortion components in the output RF signal have the sameamplitude and opposite phase relationship with respect to the non-lineardistortion components that are contained in the distortion outputsignal. As a result, one obtains the main signal components alone on theoutput terminal 17. In the present embodiment, the optimization of thefeed forward loop for cancellation of the non-linear distortioncomponents does not rely upon the minute non-linear distortioncomponents remaining in the output RF signal outputted from thedirectional coupler 13. Thereby, the optimization of the feed forwardloop can be achieved with reliability. In FIG. 21, it should be notedthat the detectors 53 and 54 as well as the control unit 55 form thesecond control unit 22B that controls the distortion elimination loop 2.

Next, the construction of the control unit 55 used in the feed forwardamplifier of FIG. 21 will be described with reference to FIG. 22.

Referring to FIG. 22, the control unit 55 includes a first distributor551 supplied with the output of the detector 53 corresponding to thesignal A for distributing the same into two signal paths and a seconddistributor 552 supplied with the output of the detector 54corresponding to the signal B for distributing the same into two signalpaths, wherein a first output signal A₁ of the power distributor 551 issupplied on the one hand to a first input terminal of a comparator 554after detection in a detector 553 and simultaneously to a first inputterminal of the comparator 555. On the other hand, a second outputsignal A₂ of the distributor 551 is supplied to a directional coupler556. The directional coupler 556 is further supplied with an outputsignal B₁ of the power distributor 552, wherein the directional coupler556 produces and supplies an output signal formed as a result ofsynthesis of the signal A₂ and the signal B₁ to a detector 557. Further,the other output signal, a signal B₂, of the power distributor 552 issupplied to a detector 558.

The output of the detector 558 is supplied to a second input terminal ofthe comparator 554, wherein the comparator 554 produces an output signalindicative of the difference in the amplitude of the signal that issupplied to the first input terminal and the signal that is supplied tothe second input terminal. The output signal of the comparator 554 isthereby supplied to the variable attenuator 14. In response to this, thevariable attenuator 14 changes the attenuation factor applied to thedistortion output signal that has been produced by the directionalcoupler 8 in response to the output of the comparator 554, such that theoutput of the comparator 554 becomes zero.

The output of the detector 558 is supplied also to the second inputterminal of the comparator 555, wherein the comparator 555 produces anoutput signal indicative of a difference between the signal A and thesignal B, similarly to the comparator 554, and supplies the outputsignal thus produced to a first input terminal of another comparator559. The comparator 559 is further supplied with an output signal of thedetector 557 at a second input terminal, and produces a signalindicative of the difference between the signal supplied to the firstinput terminal and the signal supplied to the second input terminal. Thesignal thus obtained by the comparator 559 is supplied to the variablephase shifter 15 for controlling the phase shift caused by the phaseshifter 15. The output signal of the comparator 559 is representedmathematically as |A+B|- (|A|+|B|), wherein the variable phase shifter15 changes the phase characteristics thereof such that the valuerepresented by the foregoing equation becomes zero. When the conditions|A|=|B| and |A+B|=|A|+|B| are satisfied, the respective output of thecomparators 554 and 559 become zero and the operation of the distortionelimination loop converges. As explained before, because of the factthat the control unit 55, and hence the control unit 22B, achieves thecontrol of the distortion elimination loop by comparing the signal A andthe signal B, the control operation of the feed forward amplifier of thepresent embodiment is more stabilized, compared with that of theconventional feed forward amplifier shown in FIG. 6. This advantageappears particularly conspicuous in the vicinity of the point ofconvergence.

FIG. 23 shows a seventh embodiment of the present invention, wherein itwill be noted that the present embodiment corresponds the a modificationof the embodiment of FIG. 21. Thus, those parts in FIG. 23 thatcorrespond to the parts in FIG. 21 are designated by the same referencenumerals and the description will be omitted.

In the present embodiment, the control of the distortion extraction loop1 and the control of the distortion elimination loop 2 are achieved bythe same control circuit 22. Thus, not only the distortion eliminationloop 2 but also the distortion extraction loop are controlled such thatthe foregoing conditions of convergence of |A|=|B| and |A+B|=|A|+|B| aremaintained. Because of this, the directional coupler 27 and the detector28 as well as the first control unit 22A are not used. The control ofthe distortion elimination loop 1 is achieved, for example, bycontrolling the variable attenuator 10 by the output of the comparator554 shown in FIG. 22 and by controlling the phase shifter 11 in responseto the output of the comparator 554 of FIG. 22. Although the presentembodiment cannot provide the exact adjustment of the distortionextraction loop 1 and the distortion elimination loop 2, theconstruction of the feed forward amplifier is significantly simplifiedand the convergence of the amplifier is facilitated.

FIG. 24 shows an eighth embodiment of the present inventioncorresponding to a modification of the embodiment of FIG. 21. Thus,those parts of FIG. 24 that are common with the parts used in theconstruction of FIG. 21 are designated by the same reference numeralsand the description thereof will be omitted.

In the present embodiment, the phase inverter 52 provided in theconstruction of FIG. 21 between the directional coupler 51 and thedetector 53 is removed, and a phase inverter 52' is interposed betweenthe directional coupler 23 and the detector 54. The construction of FIG.24 can also provide an operation identical with that of the constructionof FIG. 21.

It should be noted that the control unit 55 of FIG. 21 may beconstructed by a microprocessor. Thereby, the control is achieved bysoftware. In each of the embodiments of FIGS. 21-23, one can achieve areliable detection even when a low cost detector is used for thedetectors 53 and 53, because of the large input signal.

Next, a ninth embodiment of the present invention will be described withreference to FIG. 25, wherein the present embodiment relates to animprovement of the distortion extraction loop 1 used in the conventionalfeed forward amplifier shown in FIG. 1. In the present embodiment, onemay use the distortion elimination loop shown in FIG. 1 or any of thedistortion elimination loops described heretofore.

Referring to FIG. 25, the feed forward amplifier of the presentembodiment is supplied with the output RF signal from the main amplifier5 after branching by the directional coupler 6, wherein there isprovided a phase shifter 61 for causing a phase shift of +90° or -90° inthe output RF signal thus supplied. The phase shifter 61 supplies anoutput signal a to a mixer 72. On the other hand, the phase shifter 11changes the phase of the input RF signal supplied via the delay line 9by about -90° or +90°, such that there exists a phase difference of 180°between the output RF signal supplied to the directional coupler 8 andthe input RF signal.

Further, the foregoing input RF signal b that has passed through thevariable phase shifter 11 is supplied to the foregoing mixer 62, whereinthe mixer 62 carries out a synchronous detection of the signal a whileusing the signal b as the carrier. Thereby, the mixer 62 produces a d.c.output signal c. It should be noted that the output signal c correspondsto the main signal components contained in the signal a and that themagnitude of the signal c becomes zero when there exists a phasedifference of exactly 90° between the signal a and the signal b. SeeFIG. 15 explained before. Thus, by controlling the variable phaseshifter 11 such that the output level of the signal c is zero, one canset the phase difference between the signal a and the signal b to beexactly 90 degrees and simultaneously the phase difference between theoutput RF signal and the input RF signal, at the directional coupler 8to be exactly 180°. As a result, the distortion extraction loop 1 canextract the non-linear distortion components with reliability.

In order to optimize the distortion extraction loop 1 described above,the feed forward amplifier of FIG. 25 is provided with a control unit 63that is supplied with the d.c. output signal c of the mixer 62 forcontrolling the variable phase shifter 77. Of course, the control unit63 controls the phase shifter 11 such that the magnitude of the d.c.output signal c becomes zero.

According to the present embodiment, one can construct a high precisiondistortion elimination loop with a simple construction. By combining thedistortion extraction loop of the present embodiment with the distortionextraction loop described in any of the foregoing embodiments, one canconstruct a feed forward amplifier wherein the non-linear distortion iseffectively suppressed.

FIG. 26 shows the construction of a feed forward amplifier according toa tenth embodiment of the present invention. The present embodiment isan improvement of the conventional feed forward amplifier which is shownin FIG. 1, wherein it will be noted that the distortion elimination loopis cascaded in a plurality of stages.

Referring to FIG. 26, the present embodiment employs another hybridcircuit 71 between the hybrid circuit 4 and the delay line 9 fordistributing the input RF signal supplied from the hybrid circuit 4 tothe delay line 9 and to another delay line 9'. Similarly to the previousembodiments, the input RF signal that has been supplied to the delayline 9 is subjected to the extraction of distortion components in thedistortion elimination loop 1, and the non-liner distortion componentsthus extracted are used in the directional coupler 13 for canceling outthe non-linear distortion components contained in the main signal.

In the present embodiment, the RF signal thus removed with thenon-linear distortion components is not supplied to the output terminal17 directly but to another distortion elimination loop 2' of the nextstage. On the other hand, the input RF signal distributed to the delayline 9' in the hybrid circuit 71 is supplied so to pass through avariable attenuator 10' and a variable phase shifter 11', wherein theinput RF signal thus processed is synthesized with the output RF signalbranched by a directional coupler 6'. Thereby, the non-linear distortioncomponents are extracted. In FIG. 26, it should be noted that thecomponents forming the distortion elimination loop 2' are designatedwith the corresponding reference numerals of the loop 2 except for aprime symbol.

The non-linear distortion components thus extracted by the directionalcoupler 8' pass through the variable attenuator 14', the variable phaseshifter 15' and the secondary amplifier 16' and are synthesized with theoutput RF signal supplied from distortion elimination loop 2 of theprevious stage. Thereby, the non-linear distortion components remainingin the foregoing output RF signal are canceled out. Further, in order tocontrol the operation of the foregoing distortion extraction loop 1 aswell as the operation of the distortion elimination loops 2 and 2',detectors 21, 21', 28 and 28' are provided. The detector 28 is used, asalready explained with the previous embodiment, for detecting the mainsignal components remaining in the distortion output signal extracted bythe directional coupler 8, while the detector 28' is used to detect themain signal components remaining in the distortion output signal thathas been extracted by the directional coupler 8'. Further, the detector21 is used for detecting the non-linear distortion components remainingin the output RF signal extracted by the directional coupler 13, whilethe detector 21' is used for detecting the non-linear distortioncomponents remaining in the output RF signal extracted by thedirectional coupler 13' and outputted to the output terminal 17.

According to the present embodiment, the non-linear distortioncomponents remaining in the output RF signal are removed in thedistortion elimination loop 2, and the non-linear distortion componentsstill remaining after the foregoing process are removed by thedistortion elimination loop 2'. Thereby, a more complete elimination ofthe non-linear distortion components can be achieved as compared withthe conventional feed forward amplifier.

Next, an eleventh embodiment of the present invention will be describedwith reference to FIG. 27.

The present embodiment addresses the problem that may occur in theembodiment of FIG. 26 that the residual non-linear distortion componentshave so small a magnitude that the control of the loop 2', based uponthe output of the detector 21', may become unstable. In order toovercome the problem, the present embodiment employs an injection of apilot signal into the output RF signal of the amplifier 5. The pilotsignal that is injected into the output RF signal of the main amplifier5 and the pilot signal that is injected into the output RF signalextracted by the directional coupler 13 are preferably the same signaland, thus, the same pilot generator 19 is used for producing the pilotsignals. In the construction of FIG. 27, the pilot signal is injectedinto the output RF signal that has experienced the removal of thedistortion in the distortion elimination loop 2 and, and because ofthis, the problem that the pilot signal detected by the detector 21' istoo small for the reliable operation of the control system is overcome.Of course, the RF signal outputted from the output terminal 17 issubstantially free from the non-linear components.

In the embodiments of FIGS. 26 and 27, one may employ the distortionelimination loop of FIGS. 21-24.

Further, the present invention is not limited to the embodimentsdescribed heretofore, but various variations and modifications may bemade without departing from the scope of the invention.

In the feed forward amplifier the present invention, it is possible tocarry out the operation of the distortion extraction loop even when thelevel of the input RF signal has changed. Thus, the feed forwardamplifier of the present invention is particularly suitable for broadband RF terminals for use in base stations of mobile telephone networksthat include therein mobile terminals. Further, because of the fact thatthe feed forward amplifier of the present invention can achieve theoperation of the distortion extraction loop and the operation of thedistortion elimination loop independently, a quick convergence of theoperation is expected. Further, the present invention provides highprecision removal of distortions by applying synchronous detection, inthe distortion elimination loop and/or distortion extraction loop, tothe pilot signal that has been injected for the extraction and removalof the distortion. In the present invention, by carrying out thedetection of the non-linear distortion components before the removal ofthe distortion with respect to both amplitude and phase, a reliable andhigh precision control for removing the distortion is achieved. Further,according to the present invention, by carrying out the elimination ofthe distortion over a number of successive steps, one can almostcompletely remove the non-linear distortion components remaining in theoutput RF signal.

We claim:
 1. A feed forward amplifier comprising:RF amplificationmeans,.supplied with an input RF signal at an input terminal, foramplifying the input RF signal and thereby producing an output RFsignal; a distortion extraction loop, supplied with said input RF signalfrom said input terminal and further with said output RF signal fromsaid RF amplification means, for extracting, from said output RF signal,non-linear distortion components formed in said output RF signal as aresult of amplification in said RF amplification means; variable phaseshifter meanst in said distortion extraction loop, for varying a phaseof said input RF signal that has been supplied to said distortionextraction loop; variable attenuation means, in said distortionextraction loop, for attenuating an amplitude of said input RF signalthat has been supplied to said distortion extraction loop; distortionextraction means, in said distortion extraction loop and supplied withsaid input RF signal after processing in said variable phase shiftermeans and said variable attenuation means and with said output RF signalfrom said RF amplification means, for producing a distortion outputsignal that includes non-linear distortion components; distortionelimination means, supplied with said distortion output signal from saiddistortion extraction means and said output RF signal from said RFamplification means, for canceling out said non-linear distortioncomponents contained in said output RF signal by said non-lineardistortion components contained in said distortion output signal; firstcontrol means, supplied with said input RF signal that has been suppliedto said input terminal and further with said distortion output signal,for extracting a main signal component contained in said distortionoutput signal said first control means controlling said variable phaseshifter means and said variable attenuation means independently of eachother so as to decrease a ratio of a level of said main signal componentto a level of said input RF signal; said first control means furthercomprising branching means for branching said input RF signal suppliedto said input terminal, first level detection means for detecting alevel of said input RR signal that has been branched by said branchingmeans, second level detection means for detecting a level of said mainsignal component contained in said distortion output signal of saiddistortion elimination means, and a control circuit supplied with outputsignals from said first and second level detection means for controllingsaid variable attenuation means and said variable phase shifter means insaid distortion extraction means; and automatic gain control means,supplied with said output signal from said first level detection means,for controlling a level of said input RF signal supplied to saiddistortion extraction loop from said input terminal.
 2. A feed forwardamplifier comprising:RF amplification means, supplied with an input RFsignal at an input terminal, for amplifying the same and therebyproducing an output RF signal; a distortion extraction loop, suppliedwith said input RF signal from said input terminal and further with saidoutput RF signal from said RF amplification means, for extracting, fromsaid output RF signal, non-linear distortion components formed in saidoutput RF signal as a result of amplification in said RF amplificationmeans; distortion elimination means, supplied with a distortion outputsignal from said distortion extraction loop and further with said outputRF signal from said amplification means, for canceling out saidnon-linear distortion components contained in said output RF signal bysaid non-linear distortion components contained in said distortionoutput signal and producing a distortion-eliminated output signal; pilotsignal generation means for producing a pilot signal; pilot signalinjection means for injecting said pilot signal into said output RFsignal produced by said RF amplification means; pilot signal detectionmeans, supplied with said pilot signal from said pilot signal generationmeans, for detecting a pilot signal component contained in thedistortion-eliminated output signal produced by said distortionelimination means and for conducting d.c. detection by mixing said pilotsignal into said distortion-eliminated output signal and producing ad.c. detection signal as an output; and control means supplied with thed.c. detection signal produced by said pilot signal detection means,said d.c. detection signal being produced by said pilot signal detectionmeans as a result of said d.c. detection, said control means controllingsaid distortion elimination means so as to minimize the level of saidd.c. detection signal; said pilot signal detection means furthercomprising phase control means for coinciding the respective phases ofsaid pilot signal component contained in said distortion-eliminatedoutput signal and said pilot signal injected for said d.c. detection. 3.A feed forward amplifier as claimed in claim 2, wherein said phasecontrol means further comprises second pilot signal detection means formixing said pilot signal contained in said distortion-eliminated outputsignal and said pilot signal injected for said d.c. detection, saidsecond pilot signal detection means thereby producing a d.c. outputsignal, and a control circuit, supplied with said d.c. output signalfrom said second pilot signal detection means, for coinciding therespective phases of said pilot signal contained in saiddistortion-eliminated output signal and said pilot signal injected forsaid d.c. detection, such that said d.c. output signal of said secondpilot signal detection means is maximized.
 4. A feed forward amplifieras claimed in claim 3, wherein said phase control means furthercomprises automatic gain control means for maintaining a level of saidpilot signal, contained in said distortion-eliminated output signal, ata constant level.
 5. A feed forward amplifier as claimed in claim 3,wherein said phase control means further comprises automatic gaincontrol means for setting a level of said pilot signal, contained insaid distortion output signal to be coincident with a level of the pilotsignal mixed for said d.c. detection.
 6. A feed forward amplifiercomprising:RF amplification means, supplied with an input RF signal, foramplifying the same and thereby producing an output RF signal; adistortion extraction loop, supplied with said input RF signal andfurther with said output RF signal from said RF amplification means, forextracting, from said output RF signal, non-linear distortion componentsformed in said output RF signal as a result of amplification in said RFamplification means, said distortion extraction loop thereby producing adistortion output signal containing therein non-linear distortioncomponents; a distortion elimination loop, supplied with said distortionoutput signal from said distortion extraction loop and further with saidoutput RF signal from said amplification means, for canceling out saidnon-linear distortion components contained in said output RF signal bynon-linear distortion components contained in said distortion outputsignal, said distortion elimination loop further comprising: variableattenuation means, supplied with said distortion output signal producedby said distortion extraction loop, for modifying an amplitude of saiddistortion output signal, variable phase shifter means, supplied withsaid distortion output signal produced by said distortion extractionloop, for varying a phase of said distortion output signal, anddistortion elimination means, supplied with said distortion outputsignal after processing thereof in said variable attenuation means andsaid variable phase shifter means and with said output RF signal fromsaid amplification means, for synthesizing said distortion output signaland said output RF signal and thereby canceling out the non-lineardistortion components contained in said output RF signal by thenon-linear distortion components contained in said distortion outputsignal; and control means, supplied with said distortion output signaland said output RF signal, for controlling said variable attenuationmeans and said variable phase shifter means such that said non-lineardistortion components in said output RF signal have an amplitude and aphase that are identical, respectively, to an amplitude and a phase ofsaid non-linear distortion components contained in said distortionoutput signal.
 7. A feed forward amplifier as claimed in claim 6,wherein said control means further comprises:phase inversion means forinverting a phase of said output RF signal supplied to said distortionelimination loop from said distortion extraction loop; a synthesizer forsynthesizing an output signal of said phase inversion means and saiddistortion output signal supplied to said distortion elimination loopfor producing a synthetic output signal; a first detector for detectingan output of said synthesizer to produce a first detection signal, asecond detector for detecting said distortion output signal forproducing a second detection signal, and a third detector for detectingsaid output signal of said phase inversion means for producing a thirddetection signal; and a control circuit supplied with said first throughthird detection signals for controlling said variable attenuation meansand said variable phase shifter means such that said second and thirddetection signals have a common magnitude and such that a sum of saidsecond and third detection signals has a magnitude which is identical toa magnitude of said first detection signal.
 8. A feed forward amplifieras claimed in claim 6, wherein said control means furthercomprises:phase inversion means for inverting a phase of said distortionoutput signal after processing in said variable attenuation means andsaid variable phase shifter means; a synthesizer for synthesizing anoutput signal of said phase inversion means and said output RF signalfor producing a synthetic output signal; a first detector for detectingsaid synthetic signal of said synthesizer to produce a first detectionsignal, a second detector for detecting said output RF signal to producea second detection signal, and a third detector for detecting saidoutput signal of said phase inversion means to produce a third detectionsignal; and a control circuit, supplied with said first through thirddetection signals, for controlling said variable attenuation means andsaid variable phase shifter means such that said second and thirddetection signals have a common magnitude and such that a sum of saidsecond and third detection signals has a magnitude which is identical toa magnitude of said first detection signal.
 9. A feed forward amplifiercomprising:RF amplification means supplied with an input RF signal foramplifying the same and thereby to produce an output RF signal; adistortion extraction loop, supplied with said input RF signal andfurther with said output RF signal of said RF amplification means, forextracting non-linear distortion components formed in said output RFsignal as a result of amplification by said RF amplification means, saiddistortion extraction loop thereby producing a distortion output signalcontaining said non-linear distortion components; and a distortionelimination loop, supplied with said distortion output signal from saiddistortion extraction loop and further with said output RF signal fromsaid RF amplification means, for canceling out said non-lineardistortion components contained in said output RF signal by saidnon-linear distortion components contained in said distortion outputsignal, 1 said distortion extraction loop further comprising:variablephase shifter means, supplied with said input RF signal, for changing aphase thereof; phase shifter means, supplied with said output RF signalfrom said RF amplification means, for causing a 90° degree phase shifttherein, synchronous detection means, supplied with said input RF signalafter processing in said variable phase shifter means and further withsaid output RF signal as processed by said phase shifter means, forsynchronously detecting same and thereby detecting main signalcomponents contained in said output RF signal; control means, suppliedwith said main signal components detected by said synchronous detectionmeans, for controlling said variable phase shifter means such that saidmain signal components are minimized; and distortion extraction means,supplied with said input RF signal after processing by said variablephase shifter means and further with said output RF signal as producedby said RF amplification means, for canceling said main signal containedin said input RF signal and main signal components contained in saidoutput RF signal, said distortion extraction means producing an outputsignal essentially consisting of said non-linear distortion components.10. A feed forward amplifier as claimed in claim 9, wherein saidsynchronous detection means comprises a mixer for mixing said input RFsignal, as supplied thereto, and said output RF signal and thereby fordetecting said main signal components, said control means controllingsaid variable phase shifter means such that main signal componentscontained in an output signal of said mixer become zero, said controlmeans thereby controlling said main signal in said input RF signals andsaid main signal components in said output RF signal to have respective,opposite phases in said distortion extraction means.
 11. A feed forwardamplifier comprising:RF amplification means, supplied with an input RFsignal from an input terminal, for amplifying the same and therebyproducing an output RF signal; a plurality of distortion extractionloops, each supplied with said input RF signal from said input terminaland further with said output RF signal of said RF amplification means,for extracting, from said output RF signal, non-linear distortioncomponents formed in said output RF signal as a result of amplificationmeans, each of said distortion extraction loops producing acorresponding distortion output signal containing therein saidnon-linear distortion components; a plurality of distortion eliminationloops, provided in correspondence to said plurality of distortionextraction loops, said distortion elimination loops being cascaded witheach other and supplied with said output RF signal from said RFamplification means, each of said distortion elimination loops beingfurther supplied with a corresponding distortion output signal from acorresponding distortion extraction loop for removing said non-lineardistortion components from said output RF signal supplied thereto; and aplurality of distortion elimination loop control means, respectivelyprovided in said plurality of distortion elimination loops and eachthereof supplied with said output RF signal from which said non-lineardistortion components are eliminated, for controlling said respectivedistortion elimination loop such that a level of said non-lineardistortion components remaining in said output RF signal is minimized.12. A feed forward amplifier as claimed in claim 11, wherein an outputRF signal is supplied to said distortion elimination loop incorrespondence to each of said plurality of distortion eliminationloops.